1. Field of the Invention
The invention relates to a method and heart rate monitoring equipment for heartbeat detection on the basis of a heart rate signal.
2. Brief Description of the Related Art
A heart rate monitor is a device used in sports and medical science, by means of which it is possible to monitor heart functions, such as rate, standard deviation of heartbeat intervals or other corresponding heart rate information. With reference to FIG. 2, heart rate monitors are constructed in such a manner, for instance, that they comprise an electrode belt 202 which is placed on the chest of a person 200 and measures the heart rate, the heart beats measured by the belt being transmitted wirelessly or through a conductor to a receiver unit 204 strapped to the wrist. Further, heart rate monitors can be constructed in such a manner that the entire heart rate monitoring equipment 204 is strapped to the wrist, from which the heart rate is measured using either an electric signal generated by a heart beat or changes in blood vessel pressure. The heart rate monitoring equipment can also be located in its entirety on the chest, in which case any required displays and memory units are in an apparatus 202 located on the chest.
The operation of a heart rate monitor is based on the fact that the heart when contracting generates a series of electric pulses which can be measured everywhere in the body, thus also in the hands and fingers. The signal is called an EKG signal. It is possible to identify sections of the different functional cycles of the heart from-this signal, as shown in FIG. 1. The figure shows the electric signal 100 generated by one heart beat on a time-signal strength level 108. The Y axis shows the signal strength (P) 106 as a function of the time (T) 104 shown on the X axis. The signal can be divided into peaks, 102A to 102E, of which the cardiogram feature that is easiest to distinguish, the QRS complex (102B-102C-102D), is generated by the contraction of the ventricles. The time instant corresponding to an R peak 102C of the cardiogram 100 is marked at time instant 104A.
The amplitude of a typical EKG signal is 1 to 2 mV measured from the chest at a good electrode contact, but if the skin is dry, it can be as low as 200 xcexcV. Measured from a finger or hand, the signal is much weaker than this. The most reliable heart rate measurement is obtained by QRS complex detection from an EKG signal, because it contains the highest amount of energy and its spectrum differs sufficiently from the spectrum of movement artefacts. When the heart rate increases as a result of strain, the duration and amplitudes of the features of an EKG signal remain nearly unchanged in a healthy person and only the distance from a P wave to the next P wave becomes shorter. The EKG spectrum contains the most significant frequency components from 2 Hz to approximately 20 to 30 Hz. The peak of the QRS complex is in the frequency range of 10 to 15 Hz. Most disturbances in heart rate measurement are caused by movement artefacts, and to minimize these disturbances, the electrode contact must be appropriate and the electrode material must be selected correctly. The spectrums of P waves, T waves and movement artefacts are in the frequency range of 1 to 5 Hz, which is below the frequency range of the QRS complex and thus easy to filter away. Disturbances caused by muscle movement disturb the measurement, because the spectrum of an EMG signal partly overlaps that of the EKG signal. EMG disturbances can be eliminated or significantly reduced by filtering the frequencies in question away from the EKG spectrum and by moving the electrodes away from large muscles which do not exist in hands. 50 Hz (60 Hz) network interference only occurs outdoors in the immediate vicinity of high-voltage lines. Indoors, network interference may disturb the measurement in hospitals and sports halls, for instance. This interference can be reduced with a band-stop filter without attenuating the frequency components of the actual EKG signal.
The heartbeat signal measured by wrist-held heart rate monitoring equipment from an electric signal or pressure pulse of the heart is thus weak and disturbed by electric signals generated by muscle movement. Present apparatuses are trained by means of a computer program, for instance, to identify the format of a QRS complex. The heart rate monitor must first collect the EKG signal of the user and the data is then downloaded into a computer. On the computer, a QRS complex is selected from the EKG signal and transmitted back to the heart rate monitor as a QRS model. Prior art solutions contain significant drawbacks. Known heart rate monitors which measure heart rate from the wrist require computer operations and additional work when the QRS complex is separated from the EKG data. Another drawback is that heart rate monitors require bidirectional telecommunications with the outside world.
It is an object of the invention to implement an improved method for detecting heartbeat. This is achieved by the method described in the following. It is a method for heartbeat detection measuring the heart rate signal of a person from the person""s skin in a first measuring area, measuring the heart rate signal of the person from the person""s skin in a second measuring area simultaneously with the measurement made in the first measuring area, transmitting timing information formed of the heart rate signal measured in the first measuring area to a part of a device in the second measuring area, detecting the heartbeat by means of the heart rate signal measured in the second measuring area and the timing information received from the first measuring area.
The invention also relates to an arrangement for heartbeat detection. The arrangement comprises a first measuring device for measuring the heart rate signal of a person on the skin of the person in a first measuring area, a second measuring device for measuring the heart rate signal of a person on the skin of the person in a second measuring area simultaneously with the measurement made in the first measuring area, which first measuring device comprises a transmitter for transmitting the timing information formed in the first measuring area to the second measuring device which comprises a receiver for receiving the timing information sent from the first measuring device, and which second measuring device comprises detection means for detecting the heartbeat by means of the heart rate signal measured in the second measuring area and the timing information received from the first measuring device.
The invention also relates to a heart rate monitor for heartbeat detection. The heart rate monitor comprises one or more means for measuring a heart rate signal on the skin of a person, a receiver for receiving timing information related to the heart rate signal, detection means for heartbeat detection by means of the measured heart rate signal and the received timing information.
Preferred embodiments of the invention are set forth in the dependent claims.
The invention relates to heart rate monitors which measure the heart rate of a person during the actual measurement in an area of a weak heart rate signal. In this context, an area of a weak heart rate signal refers to other areas than the chest which in this application is called an area of a strong heart rate signal. An area of a weak heart rate signal refers, for instance, to the wrist of a person, even though the invention is not limited to using the wrist as the measuring area. Measuring a heart rate signal from the wrist or a corresponding location can be done in several different ways, such as by measuring the electric signal generated by a heart beat, by measuring a pressure pulse generated by a heart beat in a blood vessel, or by monitoring the changes in the transmission of light in tissue caused by blood circulation. In electric measurement, a first sensor, i.e. electrode, of the heart rate monitor is held against the left hand, for instance, and one of the right hand fingers is made to touch a second electrode to achieve a potential difference. The basic principle of the invention is that the heart rate monitor is trained to identify from a heart rate signal measured in a weak heart rate signal area a signal peak, for instance an R peak of a signal, using timing information measured in a strong heart rate signal area. The measurement in the strong heart rate signal area takes place simultaneously with the measurement in the weak heart rate signal area, and timing information refers here preferably to a peak in a heart rate signal, such as an R or S peak, but the timing information may also comprise a combination of the time instants of several peaks, for instance. In such a case, information is sent from the strong heart rate signal area to a part of a device in the weaker heart rate signal measurement area always when an R peak, for instance, is detected in the EKG signal. It is then possible to locate the R peak from the measured signal in the weaker heart rate signal area. If the measurement in the weak heart rate signal area is made as a pressure measurement or optically, a maximum point, for instance, is identified in the pulse, the maximum point being an R peak corresponding to an electric signal. The method described above, in which a heart rate monitor used in a weak heart rate signal area is trained to interpret the heart rate signal, is called a training phase in the description of the invention. A using phase refers to measuring the heart rate signal in the second measuring area without external timing obtained from the first measuring area.
According to an embodiment of the invention, a measuring wave format is formed in the weak heart rate area, i.e. the second measuring area, in such a manner that the heart rate signal is read from the received timing time instant onward and backward for a given time, and the obtained heart rate signal sequence is stored in the memory buffer. The heart rate signal sequence is read from the memory buffer and compared with a wave model selected in advance. The wave model can be selected either for an entire PQRST complex or for only a part of it. Since it is known that QRS complexes are unique and may differ from each other quite a lot, several wave models of different formats should preferably be available. The wave model can, for instance, be selected for the first time in such a manner that a wave model which provides the best correlation with the measuring wave format is selected from the wave models. Because the measuring wave format and the wave model will probably not correlate perfectly at the first time, a combination wave of the above-mentioned waves is formed by averaging said waves, for instance. After this, the wave model is replaced with the formed combination wave. This is then repeated until the correlation of the wave model and the measuring wave format is sufficiently high to make the wave model good enough to depict the heart rate signal measured in the second measuring area, and after this, the wave model is selected as the comparison wave format.
During the using phase of the heart rate monitor, the measuring wave formats are preferably compared with a comparison wave format formed as described above, and when their correlation exceeds a pre-set criterion, the measuring wave format is accepted as a heart beat.
One preferred device configuration of the invention for the training phase is to have a transmitter electrode belt on the chest and a receiver unit, i.e. a wrist gauge, on the wrist. The timing information measured by the transmitter electrode belt is transmitted inductively, optically, over a connecting line or in another corresponding manner to the wrist gauge. During the using phase, the transmitter electrode belt is no longer needed, and the heart rate is measured only using the wrist gauge, i.e. heart rate monitor. The method described above and the equipment implementing the method make it possible to optimize the detection of the heart rate signal of one user. Because the heart rate signals of different persons differ from each other, the device must be trained to detect heartbeat separately for each user.
The invention provides the advantage that heartbeat detection in a weak measuring area and thus also measuring the heart rate can be considerably improved by means of the timing information obtained during the training phase from the strong measuring area.